It is the business of many refineries and chemical plants to obtain, process and upgrade relatively low value hydrocarbons to more valuable feeds, or chemical raw materials. For example, methane, the simplest of the saturated hydrocarbons, is often available in rather large quantities either as an undesirable by product in admixture with other more valuable higher molecular weight hydrocarbons, or as a component of an off gas from a process unit, or units. Though methane is useful in some chemical reactions, e.g., as a reactant in the commercial production of methanol and formaldehyde, it is not as useful a chemical raw material as most of the higher molecular weight hydrocarbons. For this reason process streams which contain methane are usually burned as fuel.
Methane is also the principal component of natural gas, which is composed of an admixture of normally gaseous hydrocarbons ranging C.sub.4 and lighter and consists principally of methane admixed with ethane, propane, butane and other saturated, and some unsaturated hydrocarbons. Natural gas is produced in considerable quantities in oil and gas fields, often at remote locations and in difficult terrains, e.g., offshore sites, arctic sites, swamps, deserts and the like. Under such circumstances the natural gas is often flared while the oil is recovered, or the gas is shut in, if the field is too remote for the gas to be recovered on a commercial basis. The construction of pipelines to carry the gas is often not economical, due particularly to the costs of connecting numerous well sites with a main line. Transport of natural gas under such circumstances is also uneconomical because methane at atmospheric pressure boils at -258.degree. F. and transportation economics dictate that the gas be liquefiable at substantially atmospheric pressures to reduce its volume. Even though natural gas contains components higher boiling than methane, and such mixtures can be liquefied at somewhat higher temperatures than pure methane, the temperatures required for condensation of the admixture is nonetheless too low for natural gas to be liquefied and shipped economically. Under these circumstances the natural gas, or methane, is not even of sufficient value for use as fuel, and it is wasted.
The thought of utilizing methane from these sources, particularly avoiding the tremendous and absolute waste of a natural resource in this manner, has challenged many minds; but has produced few solutions. It is highly desirable to convert methane to hydrocarbons of higher molecular weight (hereinafter, C2+) than methane, particularly admixtures of C.sub.2 + hydrocarbon products which can be economically liquefied at remote sites; especially admixtures of C.sub.2 + hydrocarbons rich in ethylene or benzene, or both. Ethylene and benzene are known to be particularly valuable chemical raw materials for use in the petroleum, petrochemical, pharmaceutical, plastics and heavy chemicals industries. Ethylene is thus useful for the production of ethyl and ethylene compounds including ethyl alcohol, ethyl ethers, ethylbenzene, styrene, polyethylbenzenes ethylene oxide, ethylene dichloride, ethylene dibromide, acetic acid, oligomers and polymers and the like. Benzene is useful in the production of ethylbenzene, styrene, and numerous other alkyl aromatics which are suitable as chemical and pharmaceutical intermediates, or suitable in themselves as end products, e.g., as solvents or high octane gasoline components.
It has been long known that methane, and natural gas could be pyrolytically converted to C.sub.2 + hydrocarbons. For example, methane or natural gas passed through a porcelain tube at moderate red heat will produce ethylene and its more condensed homologues such as propylene, as well as small amounts of acetylene and ethane. Methane and natural gas have also been pyrolytically converted to benzene, the benzene usually appearing in measurable quantities at temperatures above about 1650.degree. F. (899.degree. C.), and perhaps in quantities as high as 6-10 wt. % at 2200.degree. F. to 2375.degree. F., (1204.degree. to 1302.degree. C.) or higher. Acetylene and benzene in admixture with other hydrocarbons, have been produced from methane and natural gas in arc processes, cracking processes, or partial combustion processes at temperatures ranging above about 2775.degree. F. (1524.degree. C.). Heat for such reactions has been supplied from various sources including electrically heated tubes, electric resistance elements, and spark or arc electric discharges. These processes characteristically require considerable heat energy which, most often, is obtained from combustion of the by-product gases. The extreme temperatures coupled with the low yields of higher molecular weight hydrocarbons have made the operation of such processes uneconomical.
High temperature, noncatalytic, thermal pyrolysis processes involving the conversion of methane in the presence of ethane and other hydrocarbons are well known in the art. Representative articles include: Roczniki Chemi, An. Soc. Chim. Polonorum, 51, 1183 (1977), "The Influence of Ethane on Thermal Decomposition of Methane Studied By The Radio Chromatographic Pulse Technique"; J. Soc. Chem. Ind. (Trans. and Comm.) 1939,58, 323-7; and J. Chin. Chem. Soc. (Taipei) 1983, 30(3), 179-83.
Addition of hydrogen to pyrolysis reaction mixtures is well known, see for example, pp 84-85 in "Pyrolysis Theory and Industrial Practice", L. F. Albright, B. L. Crynes and W. H. Corcoran (Ed.), Academic Press (1983).
Partial oxidation processes of converting methane to C.sub.2 + hydrocarbons are well known. In these processes, hydrogen must be removed either as water, molecular hydrogen or other hydrogen-containing species. Likewise, any other polymerization mechanism wherein methane is converted to C.sub.2 + hydrocarbon products requires a tremendous amount of energy, most often supplied as heat, to provide the driving force for the reactions. In the past the molecular hydrogen liberated by the reaction has often been separated and burned to provide the necessary process heat. This route has proven an abomination to the production of C.sub.2 + hydrocarbons, but alternate reaction pathways have appeared little better, if any, for these have resulted in the production of large quantities of the higher, less useful hydrogen deficient polymeric materials such as coke, and highly oxidized products such as carbon dioxide and water.
Typical of low temperature prior art processes are those disclosed in U.S. Pat. Nos. 4,239,658, 4,205,194 and 4.172,180 which use a regenerable catalyst-reagent. U.S. Pat. No. 4,239,658, for example, teaches a process for the conversion of methane to higher molecular weight hydrocarbons. In the process, a three component catalyst-reagent is utilized which comprises a mixture of various metals and metal oxides, particularly a Group VIII noble metal, nickel or a Group VI-B noble metal, a Group VI-B metal oxide and a Group II-A metal. The patent teaches process temperatures from about 1150.degree. to 1600.degree. F. (621.degree. to 871.degree. C.), preferably 1250.degree. F. to about 1350.degree. F. (677.degree. to 732.degree. C.).
It has also been reported in Science 153, 1393, (1966), "High Temperature Synthesis of Aromatic Hydrocarbons From Methane", that aromatic hydrocarbons can be prepared from methane by contact with silica at 1000.degree. C. (1832.degree. F.). The yield of hydrocarbons was in the range of 4.8 to 7.2 percent based on the methane used in a single pass at a space velocity of 1224 hr.sup.-1.
In the J. Chinese Chem. Soc., Volume 29, pages 263-273 (1981), it is reported that methane can be converted to C.sub.2 + hydrocarbons at temperatures of 800.degree. to 1130.degree. C. and space velocities of 3100 hr.sup.-1 or less using a metal oxide catalyst. However, the total conversion of methane, at best, is 7.5 mole percent using a thorium oxide catalyst.
Franz Fischer, reports in an article entitled: "The Synthesis of Benzol Hydrocarbons From Methane At Ordinary Pressure and Without Catalyst" (Brennstoff-Chemie, Vol. 9, pp 309-316, 1928) that methane is converted to benzol hydrocarbons by passing methane through a hot tube. In carrying out this work Fischer tested many substances for catalytic activity at temperatures ranging from 650.degree. to 1150.degree. C. and at high flow rates and concluded that the substances tested were not catalytic and not necessary. Among the substances tested were elemental iron, copper, tungsten, molybdenum, tin and carbon; and the compounds potassium hydroxide and silica gel.